The pathology of hallucinations: one or several points of processing breakdown?

This chapter describes neuropathological findings, earlier unimodal models and finally, multidimensional models concerning the pathogenesis of hallucinations. Because of the primary interest in Parkinson's disease (PD) and Lewy Body Dementia (LBD), the chapter focuses on these disorders. In PD and LBD, features of repetitive and emotionally neutral visual hallucinations are a frequent, disabling and progressive phenomenon. Multi-sensory hallucinations in which visual hallucinations are accompanied later by auditory, tactile, and olfactory hallucinations can occur, but the classic syndrome involves visual hallucinations. More recent models have attempted to integrate findings obtained in various domains and have proposed that recurrent complex visual hallucinations (RCVH) are the result of a multidimensional interactional process, the contributing factors being visual input, degree of alertness and internally or pharmacologically driven biochemical modulation. The Perception and Attention deficit model (PAD) and Hobson's Activation-Input-Modulation (AIM) model applied to RCVH definitely have extended the pathogenetic concept of RCVH.